A fully three-dimensional hydraulic fracture simulator was used to demonstrate the impact of stress changes and natural fractures on hydraulic fracture network geometry. Propagating hydraulic fractures change the local stress field, affecting the trajectory of nearby fractures propagating simultaneously (e.g. in a multi-cluster stage) or sequentially (e.g. subsequent stages in the same or other laterals). This can induce fracture complexity even in a very simple stress regime with no pre-existing natural fractures. The paper also shows that the geometry of a natural fracture system can dominate the shape of the fracture network developed during hydraulic fracturing. It is essential to simulate fracture propagation in three dimensions to capture these effects accurately.
Most hydraulic fracture simulators were initially developed to simulate the propagation of a single planar fracture from a vertical well. Their ability to model multi-stage, multi-cluster fracturing of horizontal wells is very limited because it is often added using approximations and empirical correlations. More recently, a general-purpose fully hydraulic mechanically-coupled geomechanical simulator (Damjanac and Cundall, 2016) has been used to model these scenarios. This type of simulator, based on the Distinct Element Method (DEM), captures the effects of stress changes around complex fracture networks, including both the induced hydraulic fractures and natural fractures and faults (e.g. Mack and Zhang, 2016, Maxwell et al, 2016 and Leonard et al., 2016).
In the DEM-based simulator, the fracture trajectory is limited to the geometry of the pre-defined blocks in the model. It thus becomes very computationally challenging to model the propagation of fractures for which the fracture trajectory is unknown. Some solutions have been developed for two-dimensional problems (e.g. Olson, 2008, Cipolla et al, 2011), but these methods are not easily adapted to situations with dipping beds, slip on bedding planes, significant height growth or varying material properties.
Damjanac et al (2015) have described a simulator (XSite) which can be used to efficiently model fully three-dimensional fracture propagation. The simulator was specifically developed to model realistic hydraulic fracturing scenarios, including the interaction of multiple fractures with each other and with pre-existing natural fractures.
This paper presents the application of this simulator to typical field cases including zipper-fraccing and a case in the Horn River Basin in Canada with extensive natural fractures.